1. Field of the invention
The present invention relates to an automatic seam welding apparatus, and more specifically to an apparatus for seam welding flange portions of vertically divided workpieces to make a fuel tank for a vehicle.
2. Description of the Prior Art
A fuel tank for a vehicle is generally constructed of two vertically divided workpieces. After assembling parts in the workpiece, the manufacture of the fuel tank is completed by connecting overlapping flange portions of the workpieces by seam welding. The seam welding of the vertically divided two workpieces that constitute a fuel tank is performed by using, for example, a seam welding apparatus as shown in FIGS. 8-10. Two workpieces 1 and 2 constituting a fuel tank 4 are seam welded along a weld line 3 on overlapping flange portions 1a and 2a of the workpieces as shown in FIG. 11. Disc type electrodes 5 and 6 nip the flange portions 1a and 2a of workpieces 1 and 2 at a predetermined nipping force; and the seam welding is performed by continuous resistance welding between the electrodes associated with the rotation of the electrodes.
In this seam welding, workpieces 1 and 2 are held by a work holding jig 7, and the two workpieces are rotated, together with the work holding jig in a horizontal plane by rotating electrodes 5 and 6 which continuously feed the flange portions 1a and 2a of the workpieces. At the same time, arms 8 and 9 move with a circular arc motion following the rotation of electrodes 5 and 6; and a pin 10 provided below work holding jig 7, is guided along a cam groove of a cam plate 11 which is custom made according to the shape of each fuel tank. Thus, weld line 3 on flange portions 1a and 2a of workpieces 1 and 2 is formed as a desirable formed line. Japanese Utility Model publication SHO 60-34382 discloses an automatic profile control mechanism of a seam welding apparatus similar to the above mechanism.
In such a conventional seam welding apparatus, a dedicated work holding jig 7 and dedicated cam plate 11 corresponding to the peripheral shape of each kind of fuel tank are required. In the practical manufacture of fuel tanks, the exchange of the mechanism, including cam plate 11 and pin 10, requires a fairly long time and much manpower. Therefore, in most instances, an entire seam welding apparatus is reserved for each particular shaped fuel tank so as not to require the exchange of the jig and cam plate mechanism. Therefore, different fuel tanks having different shapes cannot be manufactured by the same welding apparatus, or in the same manufacturing process. As a result, a different welding apparatus for each kind of fuel tank must be installed in a seam welding process, thereby increasing the cost and the required space for the apparatus.
Moreover, in the conventional apparatus shown in FIGS. 8-10, flange portions 1a and 2a of workpieces 1 and 2 must horizontally extend in order to seam weld them with a desirable weld line. However, this limitation on the shape of the flange portions greatly restricts the design of a fuel tank.
Furthermore, in the conventional apparatus, the position of the rotation center of lower electrode 6 is usually fixed, and upper electrode 5 is pressed downwardly toward the lower electrode at a predetermined pressure. A high quality weld is required for the seam welding portion of a fuel tank to prevent the leakage of fuel or fuel gas. To satisfy this requirement, the surfaces (peripheries) of electrodes 5 and 6 are usually cut away gradually during welding by cutting tools for the purpose of eliminating the impurities which adhere to the surfaces of the electrodes during welding. Therefore, the diameters of electrodes 5 and 6 gradually decrease as a function of the their repetitive use. Since the rotational center position or axis of lower electrode 6 is fixed as described above, the decrease in its diameter causes the formation of a gap between the electrode and a work to be welded, thereby causing deterioration of the welding quality and/or deformation of the work. To prevent such problems from occurring, an operator jacks up lower electrode 6 and adjusts the vertical position of the electrode at appropriate intervals in the conventional apparatus. However, such an adjustment conducted by the operator becomes a great burden to the fuel tank manufacturing process.
Although JP-A-62-13274 discloses an automatic seam welding apparatus wherein workpieces are held by a robot, the positional relationship between the flanges of the workpieces and electrodes in the width direction of the flanges is controlled; that is, the orbit of the electrodes on the flanges is controlled in the width direction of the flanges. However, such publication neither points out the aforementioned various problems associated with the conventional apparatus, such as one shown in FIGS. 8-10, nor discloses means for solving the problems.